stress/strain in my torque wrench--please check

First thing I'd look at is the assumption (implied by the 3EI term) that the wrench acts like an ideal cantilever.

There's also room for error in the assumption for E, since you can't be sure what sort of steel was used for the beam.

If you doubt the accuracy of your wrench, clamp it in a vise and hang some weights off it. It's better to know how it actually responds to a load rather than how it "should" respond.

First, your value for Young's modulus is the one for stainless steel. Assuming it's any alloy other than a high-nickel grade, try 210. Any grade of steel will be in between those two figures, 180 and 210, but

I am not going to do the calculations. However, your formula for area moment of inertia doesn't look right. I don't have my statics book handy but I think it should be I = pi/4*r^4.

Good luck.

OK, thanks---it's chrome-plated rusty-spotted rod so you're probably right. The change from that is in the wrong direction, though :)

Here's a great tip: just drop this expression in Google---yes, the Google search field---

and good old G responds with 29.8625457 millimeters---COOL, HEH?

I got lazy and started dropping the multiplications ('*') because 'units' allows me to, but Google requires at least some to recognize arithmetical expressions

OK, you got me there--you're good. I think I inadvertently used Iz. Still, Google sez

is 51.1929355 millimeters. Now it's too much :(

I hoped it would all balance out with the pi/4 factor, but I didn't try to do it in my head, so I really didn't know which way it would go.

OOH! I LIKE that! Thanks for the tip!

I'll get some use out of that.

Yeah, backing into it, it takes a value of 260 to make it work, and that's just not possible with any steel. In general, the Young's modulus for steel falls into a very narrow range, except for high-nickel alloys (including stainless), which are lower, and high-speed steel with tungsten, which is higher. Your torque-wrench beam is most likely a plain-carbon grade, which should be in the range of 205 - 210.

Assuming your measurements are right, I don't know where else to go with it.

Given that the dependence is on diameter^4, it wouldn't take much of a measurement error to throw it off.

Yeah, that's what I was thinking, too.